Resource Acquisition and Transport

• Tradeoffs between growing tall and branching:
  • Prioritizing branching: more stems with leaves → more photosynthetic ability
  • Prioritizing height: more advantageous for directly capturing light
• Phyllotaxy: The arrangement of leaves on a stem
  • Most plants have alternate leaf arrangement
  • Fibonacci sequence of spiraling
• Root growth and physiology is flexible and adjusts based on the environment, such as mineral transport
• Different forms of transport
  • Short-term cellular transport is apoplastic, symplastic, or transmembrane
  • Membrane transport may be active or passive; proton pumps, cotransport and gradients or ionic charges (secondary active transport), ion channels

Water Transport

• Water is necessary to keep plant cells turgid
• Efficient long-distance transport uses bulk flow: large amounts of water are driven in one direction via the use of a pressure gradient
  • Osmosis and diffusion are important in the leaf and potentially in the root; not the xylem and veins
  • Bulk flow occurs in all of the veins, including minor; tissue involves both bulk flow and diffusion
  • Water is transported via the xylem sap
  • Bulk flow follows pressure potential, not solute potential
  • Bulk flow typically happens in hollowed, dead cells (water-conducting cells); living cells, xylem does not travel across membrane
  • Bulk flow carries bulk and much faster than normal diffusion

Small Plants; Root Pressure and Bulk Flow

• Is water pushed up a plant via positive pressure or pulled down under tension (via negative pressure)?
  • Positive pressure is a minor movement that occurs in plants less than 1’ and occurs in some plants in the morning: Root pressure
  • Pulling occurs as soon as stomata are opened in the plant; describes major movement
• Root pressure: Pressure builds up in the roots, causing guttation and water to exude/squeeze out of the plant
  • Causes a pattern of water droplets to exude from the tips of leaves, at the ends of specialized veins: Guttation
  • Guttation occurs as a result of active ion accumulation during night, when the stomata are closed; this lowers the water potential of the xylem of the root sap, causing positive root pressure
• Most transport in small plants occurs as a result of mineral and ion transport from the root hair apoplast into the symplastic xylem
  • The water will move upward with the ions and xylem sap when the stomata are open, pulling up the xylem sap

Large Plants; Cohesion-Tension Hypothesis

• In larger plants (trees), water moves via the Cohesion-Tension Hypothesis (Mechanism; also known as “transpirational pull”)
  1. Transpiration (water loss) and water molecule cohesion pulls water from the shoots to the roots
  2. As a result of being pulled, xylem sap is normally under tension (negative pressure)

1. Stomata open; water vapor diffuses outside of the cells through the stomata following the difference of water potential; water potential equilibriates, and water potential within the extracellular airspace decreases
2. Water evaporates out of the cell walls/surfaces into the drier extracellular airspace
3. Air-water interface retreats; walls curve, forming menisci (meniscus)
4. Meniscus has a smaller radius, causing negative pressure potential to increase in water; meniscus exerts an increased pull on the water behind the cell wall, and pulls it from neighboring cells
5. Leapfrog effect; neighboring cells cascade before pulling on the xylem, and thus the xylem sap

• Diffusion or bulk flow is determined by the strength of the gradient until reaching the xylem; xylem pull (pull of xylem sap) is bulk flow
• Adhesion assists due to pull of gravity
• The outside/extracellular air should be highest, and the soil should be lowest water potential; if the soil was much more saline, the plant is likely to die
  • Unable to maintain passive water movement
• This process always starts and stops via the shoot leaves affected by the outside air, then ultimately affects the root xylem sap
• Xylem sap recedes or “snaps back” when a stem is cut; phloem flows out due to positive pressure, and xylem sap recedes due to negative pressure
• Stem width decreases during day due to pressure narrowing the stem, and widens during night when the water settles

Sugar Transport

• Phloem transport; this is always referred to as translocation
• Phloem sap studied in 1970s through aphid-phloem sap experiment
  • Due to positive pressure, sap is processed and exuded into aphid stylet tubes
• Phloem sap is made from waters, sugars, amino acids, hormones, and some minerals or RNA
  • The included sugar tends to be sucrose
  • “Phloem-mobile minerals” are able to travel down the plant
• No one-way transport; phloem sap always travels from a source to a sink, but the source and sink depend
  • Source: An organ that net exports sugar
    • Mature leaves performing photosynthesis and producing sugars;
    • Storage roots in season exporting sugar to support parts
  • Sink: An organ that net imports sugar; it can be a consumer or depository of sugar
    • Young leaves, stems, any roots
  • Sources and sinks can vary based on the lifespan of the plant

Pressure-Flow Hypothesis

• Self-thinning: Flowers, fruits, seeds, etc. are dropped
  • Occurs when there are more sugar sinks than can be supported by sugar sources

Phloem Loading

1. Concentration of sugars (sucrose) in sieve-tube increases, lowering solute and water potential
2. Net osmosis into the sieve-tube element
   • Does not necessarily originate from xylem; xylem is a good source, but the water osmosing in comes from any area where the water potential is higher
3. Net osmosis equilibriates and increases pressure potential\
4. Passive long-distance transport via bulk flow from the source to the sink
   • Occurs in either upward or downward movement across the plant

Phloem Unloading

• First step is always passive, and other steps may be active

1. Phloem unloads the sugars from the sieve-tube
2. Net osmosis follows outward the sieve-tube
3. Pressure potential decreases

Apoplastic Phloem Loading

• Studying apoplastic loading: active transport
  • Sugar may be loaded by symplastic loading as well
• Sugars must be moved/exported into cell walls (apoplast) from many of the protoplasts (ex. mesophyll, bundle-sheath, phloem parenchymas, companion cells; not in the sieve-tube element cell, but ends in its cell wall)
• Later the sugars may travel back through plasmodesmata and load into the companion cells or sieve-tube elements
• Heavy amounts of sucrose transported across membrane; going against concentration gradient

1. Proton pump pumps H⁺ ions into the cell wall, establishing a concentration gradient
2. Cotransporter uses concentration gradient to move H+ ions back into cell
3. Cotransporter uses gradient to move sucrose into sieve-tube element; ends in phloem loading

Phloem Unloading

• First step is always passive; passive transport across the gradient
• If following steps culminate in high concentration of fruit/sugar: increase concentration and must be active transport

Signal Transduction

… is involved in:

Tropisms

Mechanical Pressure

Thigmomorphogenesis

• responses to touch or wind; more often wind
• differences in form
• wind: thicker cell walls, more lignin

Thigmotropism

• only responses to touch
• tendrils twine around something else for support
• bean plants
• days or hours

Phototropism

• receptors process a stimuli from light waves

Etiolation

• ETIOLATION vs. de-etiolation (greening) of a vegetable: the occurrence of changes in response to exposure of light/natural daylight
  • Etiolated plants respond to stimuli that they interpret as associated with behavior underground, without light, photosynthesis, and water. They have:
    • no production of chlorophyll;
    • narrow, non-expanded leaves;
    • tall stems;
    • few roots that are short/stubby;
    • (specific to eudicot seeds) a retained hook
  • De-etiolation is known as “greening” because the plant produces organ advancements that utilize photosynthesis

Circadian Rhythms

• circadian rhythms of movement; daily and seasonal movements/responses
  • blue light is also involved, but more information on phytochrome
  • near-day responses
  • occurs even under sensory deprivation
  • free-running: no environmental cues
    • every 21 up to 27 hours
  • synchronized via entrainment with the environment
• biological clocks: internal, endogenous
  • maintained by clock genes that produce transcription factors (TF₁)
    • various factors used for different, non-repressive factors
    • one transcription factor produced will, after a time lag, repress clock genes
  • “molecular clock”
• 3 kinds of flowering responses
  1. short day plant
     • flowers if the night is longer than a critical value
     • P_FR inhibits flowering in plants until it is degraded
     • chrysanthemums
  2. long day plants
     • flowers if the night is shorter than a critical value
     • PFR stimulates flowering in plants if remaining
     • iris
  3. day neutral plants
     • flowers when physiologically ready, regardless of time of day
  • photo-reversible process
  • caused by florigen hormone
  • symplastic protein: plasmodesma, leaves (phytochrome), apical meristem

Gravitropism

• response to the pull of gravity
• shoots grow away from gravity (negative)
• roots grow towards (positive)
  • central column root cap (near vascular bundles) senses gravity
    • Statolith hypothesis: amyloplast stones sense gravity (heavy with starch); lower portion of root cell; amyloplast pressure on cell membrane; proteins sense pressure and signal determining where weight is
    • fountain effect: auxin redistributes in polar transport and more of it remains on the lower side
      • calcium also redistributed

Environmental Stresses

Drought Tolerance

• controls stomata during droughts; maintains drought tolerance
  • drying/dessication response
• stoma structure:
  • guard cells have microfibril cellulose cords wrapped radially around
  • inner walls are thicker than outer walls
  • guard cells are attached at top and bottom
• stomas open as a result of increased turgor pressure: attachment causes the guard cells to pull apart
  • also see increased solute concentration
  • environmental cues: blue-light receptors, decrease in internal concentration of CO₂ in the leaf, etc.
  • environmental cue transduced → outwards H⁺-ATPase proton pump
    • creates an electrochemical gradient
  • potassium channels open; potassium enters the cell passively
  • cotransport uses the electrochemical gradient: H⁺ enters passively, chlorine ions enter through energy
  • chloroplasts that create starch in the guard cell increase solute concentration; water moves in passively
• stomas close due to drought stress
  • ABA docks to receptors of guard cells
  • signal transduction → proton pump stops immediately
    • H⁺ stops moving; electrochemical gradient ends
  • proton and chloride channels
    • hydrogen and chlorine ions exit; decrease solute concentration

Flooding

• roots are unable to get oxygen for aerobic respiration
• ethylene produced → cell death in roots
  • forms canals for oxygen to travel from above flood to roots

Salt

• drying/dessication; evaporation; minerals left behind from evaporation → soils are left with toxic materials
• salt stress
• sodium toxicity
• ions disrupt water potential gradient
• more solutees in central vacuoles
• salts are taken in and excreted on leaves

Nastic Responses

• quick responses
• not growth responses
• movement of water, changes in turgor pressure across entire leaves
• slower action potentials that involve ions on the cell membrane; connected by plasmodesmata
  • membrane depolarization; negative internal positively charges or zeroes; certain ions cross membrane
  • depolarization propagates across the membranes/cells
  • swollen pulvinus on the base of a leaf; changes in turgor prressure end propagation

Growth Hormone Responses

Apical Dominance

• Control of branching of stems via the dominance of the terminal bud
• Several hormones at play:
  • Sucrose
    • found in: shoot tip
    • purpose: controls growth of axillary buds
  • Auxin
    • found in: shoot tip
      • highest concentrations are near the shoot tip
    • purpose: indirectly inhibits bud growth via strigolactone production in the axillary buds
  • Strigolactone
    • found in: axillary buds
    • purpose: directly inhibits bud growth
  • Cytokinin:
    • purpose: stimulates bud growth
      • reverse concentrations of auxin

Acid-Growth Hypothesis

• environment conditions for AGH to occur:
  • increased turgor pressure
  • loosened cell wall
• conditional steps:
  • auxin binds to the receptors in the membrane and activates the proton pumps
  • hydrogen ions are pumped into the cell wall, increasing the acidity
  • electrochemical gradient generated; anions are influxed/permeate; more solutes in cell
    • increases turgor pressure
  • acid conditions activate expansins that move down on the wall and disrupt/remove the hydrogen bonds and enzymes that cleave hemicellulose

Seed Dormancy

• promotes seed dormancy — slowing or stopping growth
  • compare ratio of ABA to other growth promoting hormones
  • dormancy in hibernation, drought/extreme conditions
  • as a seed matures, it dehydrates/dessicates;
    • sensitive to dessication; aba increases to slow maturation
    • aba increases 100x compared to beginning of maturation
  • ABA promotes production of proteins that prevent dessication
• dormancy is stopped by inactivating or removing ABA and producing more GA; many methods
  • light cue inactivates ABA
  • cold cue inactivates ABA
  • directly washing away/removing ABA (seen in desert plants; flash floods interacting with seed coats)

Senescence

• age-induced decline
  • programmed senescing (removal)
  • everything is removed before abscission occurs
  • programmed cell death
• occurs after a burst of ethylene

Abscission

• Over time, auxin concentration decreases
• Petiole has reduced auxin, which makes it more sensitive to ethylene
  • 2 zones on the petiole:
    1. protection zone preventing infection
    2. abscission zone where cell walls weaken and leaf detaches
       • outside of the protection zone

Fruit Ripening

• Positive feedback loop of ethylene production
• Accompanied by various effects:
  • Enzymes digest cellulose
  • Chlorophyll is degraded and exported
  • Starch digests into sugar and sweetens
  • Colors and aroma change
• “One bad apple spoils the barrel”
  • A damaged apple produces ethylene and will ripen nearby apples quickly due to positive feedback loop
  • Ethylene-absorptive bags prevent cross-fruit ripening

Stem Growth

• Auxin class causes cell elongation or growth in a direction in response to light
  • promotes the elongation of the coleoptile molecules
  • derived from words to increase
  • shoot architecture/phyllotaxy
  • activates vascular cambium from dormancy
• GA causes:
  • stem elongation via cell elongation and cell division
  • stem bolting
    • rosette form of leaves; shortened internodes creating a spiral, flowered effect
      • lettuce
    • bolting causes internodes to lengthen when necessary to flower
    • rapid internode expansion
  • fruit growth and internode growth
    • preventing disease
  • germination
    • cereal (grass) seeds
    • aleurome inside the seed absorbs water
    • GA is excreted and diffused acrross endosperm into aleurome
    • aleurome response produces alpha amylase
    • alpha amylase digests starch into sugars; sugar is used to fuel growth

Steps in Signal Transduction

1. RECEPTION: a sensory cell receives an external stimulus/extracellular signal (ex. light, touch, electronegativity)
   • requires the presence of a receptor: a protein that undergoes a shape change in response to a specific stimulus (ex. potato phytochromes)
   • the receptor is typically but not always in the cell membrane
2. TRANSDUCTION: a hormone is released through the body
   • changes the form of the information and amplifies it: cause a large response to small signals
   1. release of relay proteins (kinases)
      • kinases: enzymes that phosphorylate other enzymes and activate them, causing a positive cascade
      • called relay proteins and are generally kinases
   2. release of second messengers
      • usually either ions (Calcium ions) or small molecules (cyclic GMP, cGMP) that have been stored somewhere and are released during transduction
      • cause something else to occur
   • usually both processes occur, but the occurrence of either causes a response
3. RESPONSE: effector/response cells receive the hormone and process an internal change
   • THREE notable forms of cell responses
     1. altering gene expression / altering transcription
     2. post-translational modifications
     3. alteration of membrane transport / transport across the membrane

Important Signal Hormones

• Plant hormones affect target cells: the target cells may be the same or different cells in plants
• Plant hormones may be recognized as plant growth regulators; PGR tends to reference hormones that are produced in unusually large amounts
• Five main hormones in plants: Auxin class, Cytokine class, Gibberellins class, Abscicic Acid, Ethylene

Abscisic Acid

• Abscisic acid was discovered via… studying abscission in plant parts
  • ABA has nothing to do with abscission, but the name was retained
• Abscisic acid forms… acronymized to ABA
  • Single hormone, not a class
• Abscisic acid is produced in… all plant parts
• Abscisic acid is transported in… all vascular tissue

Auxin class

• Auxin forms… A class of hormones
  • Multiple types within the class…
    • IAA: Indoleacetic Acid
      • most common auxin
      • naturally occurring
    • IBA: Indolebutyric Acid
    • Synthetic Auxins
• Auxins are produced in…
  • always the shoot tips in the apical meristems & adjacent tissues
  • additionally in young leaves and seeds
• Auxins are transported from…
  • the shoot tips to the root tips via polar transport, assistive diffusion
    • transported at 1 cm/hr: this is too fast for effusion and too slow for xylem water-conducting cells
    • transports through parenchyma cells within the xylem
    • transporters are only based in the basal cell membrane (lower cell membrane); maintained in the lower area and goes much faster than standard diffusion
• Auxins are practically used for…
  • Root-Gel/Rooting Hormone
    • Uses IAA/IBA from adventitious roots in stem cutting
    • Causes propagation
  • Eudicot Weed Killer
    • Uses synthetic 2,4-D

Cytokinins class

• Cytokinin forms… a class of modified adenine nucleotides
  • Zeatin: A natural cytokinin
• Cytokinins are produced in… roots
• Cytokinins are transported… upwards towards xylem sap to the tip of the plant
• used in cell division/control of division
  • tissue culture in parenchymal explants
  • anti-aging/preservative of leaf
• callus from equal amounts of auxin and cytokinin: a mass of undifferentiated cells
  • auxin increase → root stimulus
  • cytokinin increase → shoot stimulus
  • must have both present to form callus and differentiate

Ethylene

• Ethylene was discovered via… plants near street lamps that had a different form than plants not near the street lamps
  • Street lamps were fueled by coal gas
• Forms of ethylene… C₂H₄
  • Single hormone, not a class
• Ethylene is produced in… All plant parts
• Ethylene is transported via… diffusion through air
• Ethylene is produced by plants in response to…
  • Any kind of stress: flooding, drought, mechanical (pressing on plant/resistance), injury, infection;
  • Fruit ripening;
  • Cell death;

• triple response in pea seedlings
  • stem elongation decreases
  • stem thickens
  • stem grows laterally/horizontally

Gibberellins class

• Gibberellins was discovered in… the 1900s via rice plants with foolish seedling disease
  • secreted Gibberella fungal chemical
• Forms of Gibberellins… Gibberellic acid (GA)
  • 100s of naturally occurring GAs; GA_x
  • GA₁ is most prevalent and most effective
• Gibberellins is produced in…
  • mainly in young roots and leaves
  • additionally in seeds

Other Signaling Hormones

• These are not the largest/most important/well-known five;
• These are other classes modernly discovered

Brassinosteroids

• Forms of brassinosteroids… Steroid hormones in plants
• Brassinosteroids are produced for…
  • Similar use-cases to auxin

Jasmonates

• Forms of jasmonates… Jasmonates & methyljasmine
• Jasmonates are produced for…
  • Plant defense

Strigolactones

• Strigolactones were discovered via… Striga asiatica, a parasitic witchweed that preys on Afro-Asian cereal/plant crops
• Strigolactones are transported via… Xylem (xylem-mobile)
• Strigolactones are produced for… many roles
  • Apical Dominance

Photoreceptive Proteins

• Various signals for bending towards light; light receptors
• photoreceptors for blue light, red light

Blue Light Receptors

Cryptochromes

• Cryptochromes are produced by plants to engineer…
  • Multipurpose
  • How chloroplast moves in plants and other organisms

Phototropins

• Phototropins are produced by plants to…
  • Respond to blue-light phototropism
  • Affect coleoptile curvature

Red Light Receptors (Phytochrome)

• Forms… Two forms: detecting red light and far red light
  • One molecule is comprised of 2 identical subunits:
    • Each subunit is made of a polypeptide and a light absorbing part:
      • Light-absorbing part is identical;
      • Polypeptide has two different forms and is photoreversible:
        • Phytochrome Red (P_R)
          • Red light peak absorption: 660nm
        • Phytochrome Far-Red (P_FR)
          • Far-red light peak absorption: 730nm
• Phytochrome is produced by plants in response to…
  • Etiolation
  • Seed germination

Plant Defenses

• Plants need to defend against:
  • Herbivores (Plant Eaters): Rabbits, deer, insects
    • Nematodes: Soil-dwelling roundworms
  • Pathogens (Disease-causing viruses and organisms) bacteria, fungi
  • Unable to move
• Two kinds of defenses: constitutive and induced

Constitutive Defenses

• Constitutive defenses… which are always present, but expensive

1. at the molecular level: secondary chemicals/secondary metabolites
   • second metabolites: molecules that are closely related to key compounds involved in synthetic pathways, but are not universal to plants
   • opium poppy sap produces opium, morphine, codeine
     • these compounds, as well as others such as caffeine, nicotine, and tetra-hydrocannabinol disrupt nervous systems in insects and vertebrates
   • chrysanthemum pyrethrins/pyrethoids
     • aromatic phenolics
     • Conium maculatum (hemlock) coniine produces alkaloid poisons
   • flavorful oils (peppermint, lemon, basil, sage) are insect-repellent
   • pine/fir pitch has toxic pinene
   • many have tannin molecules; similar to proteinase inhibitors
2. at the cellular level: sharp raphide crystals that affect biting
3. at the tissue level: dense lignin
4. at the organ level: modified pointy defenses made from tough tissue such as spines, thorns, deceptive butterfly eggs

Pathogen-Induced Defenses

• Pathogen-induced defenses… induced by the presence/attack of pathogens; made as necessary
  • Part of an evolutionary arms race; ~50% human food supply wordlwide has been reduced due to plant diseasee
• Pathogens tend to enter through stomata or wounds
  • The epidermal cells are covered by a waxy cuticle, as well as constitutive defenses
  • Pathogens attempt to breach surface and grow in the apoplast, then kill host cells and consume debris or nutrients

Hypersensitive Response

• **A local response
• A quicker response: takes a few hours
• Symptoms include:
  • Stomatal closure;
  • Producing targeted pathogenic toxins;
  • Increasing the strength/size of neighboring cells; walling off;
  • Programmed cell death
• Similar to cell-mediated immune response

Systemic Acquired Resistance (SAR)

• A systemic, plantwide response; occurs after hypersensitive response has begun
  • Same signal leads to MeSA response
• A slower response: takes several days
  • Releases an alarm hormone… Methyl salicylate (MeSA)
    • In the same class as aspirin
    • Methyl salicyate transports… through the phloem
  • Cell responds to methyl salicate signal by… up-regulating gene transcription for pathogen-related defenses

Herbivore-Induced Defense

• proteinase inhibitors bind to proteinase enzymes in herbivores’ mouths and digestive tracts and digest proteins
  • normally present to a lower degree in plants before induced
• causes herbivores to get sick

Systemin

• in scenarios of mechanical wounding caused by herbivores that takes several days
• small polypeptide produced: 18 amino acids: systemin
  • first peptide hormone described in plants
• systemin is released from damaged cells; then travels through phloem to target cells and causes production of jasmonic acid
  • Jasmonic Acid activates genes for 15+ herbivore-related defenses including proteinase inhibitors
• systemin signal results in the buildup of potent concentrations of insecticides in tissues that are in imminent danger

Methyl Jasmonate

• volatile
• other plants can pick up this signal regardless of species and defend pre-emptively against herbivores; some plants call for help from other organisms
• “Talking trees” hypothesis and “eavesdropping”

Pheromones

• attracts non-herbivore insects; parasitoids
  • often wasps
  • free-living adults that are parasitic as larvae
  • parasitizing other insects and laying eggs; develop larvae that eat frrom inside out